This application is based on and claims priority from Japanese Application Nos. HEI. 3-252686 filed Jun. 26, 1991 and HEI. 4-101384 filed Apr. 21, 1992, the disclosure of which is incorporated by reference herein.
The present invention relates to a scanning optical system such as a laser plotter that draws an image by scanning with laser light. More particularly, the present invention relates to a scanning optical system that is capable of automatic focusing on the image plane.
The conventional laser plotters are intended to draw images of comparatively low precision and the spot diameter which determine the feature size (minimum line width) has been about 30 .mu.m. In order to draw images of higher precision and smaller feature size, the spot on the image plane must be further reduced. Since the spot diameter is proportional to the F number of the optical system, it can be reduced by diminishing the latter.
However, the depth of focus of the optical system is proportional to the square of its F number, so if the latter is reduced with a view to improving the precision of patterning or image, it is highly likely that the image plane is out of the depth of focus on account of various factors such as the waviness and tilting of the plane.